1. Field of the Invention
The present invention relates to a planar light source device for use in a liquid crystal display (LCD), and particularly to a light emitting diode used in a planar light source device having a diffraction grating.
2. Prior Art
A typical LCD device comprises a liquid crystal display panel, and a surface lighting device mounted under the liquid crystal display panel for supplying light beams thereto. The surface lighting device mainly comprises one or more light sources for emitting light beams, and a light guide plate. The light guide plate receives the emitted light beams via a light incident surface thereof, and emits the light beams via a light output surface thereof in order to uniformly illuminate the liquid crystal display panel.
Conventionally, there are two types of light sources used in a surface lighting device of an LCD: a linear source such as a cold cathode fluorescent lamp (CCFL), and a point light source such as a light emitting diode (LED). In general, it is more difficult to obtain uniform illumination when using a series of point light sources compared with using a linear source, due to the divergent angles of light emitted from the point light sources. For example, LEDs used as light sources emit light beams having a divergent angle in the range from 30° to 130°. FIG. 5 shows a light beam distribution of a planar light source device using LEDs as the light sources. The planar light source device comprises a light guide plate 13, and a plurality of LEDs 12 arranged at one side of the light guide plate 13 for providing light beams. In operation, light beams emitted by the LEDs 12 enter the light guide plate 13 through a light incident surface (not labeled) thereof, and then transmit out from a light output surface (not labeled) thereof. Thus, the light guide plate 13 generally exhibits a plurality of dark areas 15 corresponding to spaces between adjacent LEDs 12. It is difficult to obtain uniform illumination for the planar light source device.
It is desired to provide a planar light source device for use in a liquid crystal display device which overcomes the above-described problems.